The processes controlling the rate of Na reabsorption by the toad urinary bladder epithelium will be studied, with particular emphasis on the factors governing apical membrane permeability. The possible role of Na selective channels in the apical membrane will be assessed, using fluctuation analysis to measure the turnover rates, temperature dependence and saturation properties of individual transport sites, and amiloride-sensitive currents to quantify the ion-selectivity of the sites. The regulation of membrane transport sites by the hormones aldosterone and vasopressin, and by alterations in cellular metabolism will also be studied. The effects of hormones and metabolism on the number, turnover rate and dynamics of the putative channels will be determined, and the possibilities of hormonal regulation of channel activation and channel biosynthesis explored.